OFDM

[Ranjeet Kumar]

Hi All,
we can calculate the subcarrier frequencies. The subcarrier frequencies are integer multiples of 1/Tsymbol, where
Tsymbol is the duration of an OFDM symbol.
The pdf uploaded here is that of OFDM modulator, In this file i am unable to understand why the subcarrier frequencies are 1.25 ghz, 2.50 ghz, 3.75 ghz. I request you all to please explain how i will get duration of OFDM symbol.

Attachments:

1. ofdm.pdf

[Gokul Boro .]

Hi Ranjeet, the subcarrier frequencies are as you have stated above because the frequency spacing between two consecutive subcarriers is 1.25 GHz. Since in OFDM the subcarriers are numbered like 1,2,3,…. Now if we multiply each of them by frequency gap, we get 1.25 ghz, 2.50ghz,…

[FAYIQA NAQSHBANDI]

HELLO RANJEET
Well i find the comment posted by gokul boro very useful in this regard. If you see the the frequency spacing between two consecutive subcarriers is 1.25 GHz and he’s right that the subcarriers are numbered like 1,2..so on . By multiplying the each of them by frequency gap, we get 1.25 GHZ 2.50 GHz respectively. I feel this should be enough for you to understand.
Thanks and regards

[SAHIL SINGH]

Hi Ranjeet

I completely agree with both Fayiqa Naqshbandi and Gokul Boro.The frequency spacing between two consecutive subcarriers is 1.25 GHz and the subcarriers are numbered like 1,2..so on . By multiplying each of them by frequency gap, we get 1.25 GHZ 2.50 GHz respectively. Hope this helps.

Thanks and regards

Sahil Singh

[Mohamed Ben zegalam]

Hi Ranjeet,

I agree with previous statements from Fayiqa Naqshbandi, Gokul Boro, and SAHIL SINGH. but first you need to know how to calculate the frequency spacing, then the subcarrier frequency will be as mention before

frequency spacing (dalta F)= Bit rate/(log2(M)*Nsc)

where the M is the M-array of the modulation, and Nsc is the number of subcarriers

hopefully this help you.

Kind regards
Mohamed

[alistu]

Thank you Mohamed and Gokul. It was indeed useful.

[Ranjeet Kumar]

Thank you for your considerable response,
Earlier i was unable to understand properly the topic being discussed.
I think the answer will be as:
possible
For examples if the global bit rate is 40Gbit/ and a QPSK modulation (2 bits per symbol), the symbol rate will
be 20Gsym/s. In both cases there are a total of 128 sub-carriers available such that:
OFDM symbol period = Symbol period * 128 = 128/(Symbol rate) = 2 * 128 /Global bit
rate.

[Mohamed Ben zegalam]

Hi Ranjeet,

Your example is correct, and it is the same as my explanation. because

OFDM symbol period= 1/frequency spacing (dalta F).

so if you take frequency spacing (dalta F)= Bit rate/(log2(M)*Nsc), then

OFDM symbol period= (log2(M)*Nsc)/Bit rate, as per your example

OFDM symbol period= (2*128)/40Gbps

Kind regards
Mohamed

[SAHIL SINGH]

Hi Ranjeet,

You are absolutely right.. It seems that you got the desired help you were looking for…

Regards

Sahil Singh

[Ranjeet Kumar]

Hi Mohammad Ben,
acoording to you, the frequency spacing, then the subcarrier frequency will be as mention before
frequency spacing (dalta F)= Bit rate/(log2(M)*Nsc) where the M is the M-array of the modulation, and Nsc is the number of subcarriers
for bit rate of 10 gbps and no. of subcarriers = 4, and M = 4, we will get delta F = 10ghz /[(log2{4})*4] = 1.25 ghz;
You really make me understand, it may also be another explanation.
Thank you.

[Gokul Boro .]

Thanks to everyone for participating in this discussion. It was indeed a useful discussion for me.
I want to know if there is any formula for calculation of OFDM bandwidth after OFDM block (I & Q separately) as well as One after RF-upconversion.

[Ranjeet Kumar]

Thanks Mohamed Ben zegalam, sahil singh and Gokul for your approval and considering my responses.
It was really a nice and healthy discussion. thank you all

[MHD NA]

If you see the the frequency spacing between two consecutive subcarriers is 1.25 GHz and he’s right that the subcarriers are numbered like 1,2..so on . By multiplying the each of them by frequency gap, we get 1.25 GHZ 2.50 GHz respectively.

[Rajguru M. Mohan]

Hi Ranjeet,
I think the answer will be as possible
The subcarrier frequencies are integer multiples of 1/Tsymbol, where Tsymbol is the duration of an OFDM symbol.
And in OFDM the component allows the introduction of a cyclic extension of the symbol transmitted
or a guard time with zero transmission. The parameter Number of prefix points will
define how many points will be used in the guard period.
For examples if the global bit rate is 40Gbit/ and a QPSK modulation (2 bits per symbol), the symbol rate will
be 20Gsym/s. In both cases there are a total of 128 sub-carriers available such that:
OFDM symbol period = Symbol period * 128 = 128/(Symbol rate) = 2 * 128 /Global bit
rate.

Thanks,

[Author]

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